Originally subnets were designed to cover only one availability domain (AD) in a region. They were all AD-specific, which means the subnet's resources were required to reside in a particular availability domain. Now subnets can be either AD-specific or regional. You choose the type when you create the subnet. Both types of subnets can co-exist in the same VCN. In the following diagram, subnets 1-3 are AD-specific, and subnet 4 is regional.a

Aside from the removal of the AD constraint, regional subnets behave the same as AD-specific subnets. Oracle recommends using regional subnets because they're more flexible. They make it easier to efficiently divide your VCN into subnets while also designing for availability domain failure.